This invention relates to a phase agile antenna for use in determining the direction of a radio frequency signal and ranging to achieve position determination.
In the prior art there are may methods for determining direction and range to a signal source. For example, in U.S. Pat. No. 4,339,755 to Wright, a pair of antennas spaced by a known distance apart utilizes receiver signals from a source, the antennas being arranged such that signals are received by one antenna before the other antenna due to different travel distances for the signal to each antenna. A triangulation system is then utilized to determine range to the source and direction to the source. In Zablotney et al U.S. Pat. No. 5,054,450, a similar system is used with a single antenna. Hipp et al U.S. Pat. No. 5,321,410 disclose a system in which an antenna array in a circle around a large through mast has direction capabilities enhanced where measurements of amplitude and relative phase shift show an induced distortion pattern through the mast. Analysis of the amplitude and phase measurements determine the angle of incidence of the incoming signal. In Murphy et al U.S. Pat. No. 5,541,608, a hybrid amplitude phase comparison direction-finding system for aircraft is disclosed. In that system, a calibration system utilizes the programmable read-only memory for each antenna pair where calibration and data from premeasured antenna patterns of their associated antennas are stored in their prom. A system processor retrieves the calibration data during system operation to arrive at the angle of measurements before the direction-finding aspects.
According to the present invention, a direction-finding system includes a referenced antenna and array of antenna positions around the reference antenna for receiving digital telephony radio signals at a base station from a mobile source. Phase frequency and amplitude detection of the relative phases of the radio signals received at the reference antenna and at each of the antenna positions in the circular array are used to derive the direction to the source relative to the reference antenna.
Range Determination
In conventional radars, range estimation ("ranging") is done by determining the time required for a pulsed signal to reach the "target" and subsequently for its echo to return to the radar receiver. This time is multiplied by the velocity of light and radio waves (approximately 186,000 miles per second) to determine two-way range, or halved to find one-way range.
The invention applies to cellular telephone systems that communicate using digital modulation, such as Code Division Multiple Access communications. In this signaling method, digitally coded information modulates a repetitive pseudo noise (PN) pulse train whose transitions ("chips") occur at a much faster rate than the digital information, the result being that the bandwidth required is "spread" to be much broader than the original data bandwidth. Many transmission channels can be defined within the same frequency band, if for each the transmitter uses a distinct PN pulse sequence, and if the different sequences all exhibit low correlation with one another for every time difference between the sequences. Correlation contributes to mutual interference between CDMA transmitters that operate concurrently within the same frequency band and (antenna coverage) cell.
To receive and convert information-containing pulsed signals from a CDMA transmitter, a CDMA receiver must know the precise PN sequence and Walsh codes used by the transmitter and must synchronize itself to the received sequence such that it creates an identical sequence, delayed in time by the transmission delay. The receiver combines its locally created sequence with the received signal (for example, by exclusive-OR followed by logical inversion), to extract data from the received signal. Through this "de-spreading", as is well known, the desired signal is enhanced, while noise and non-correlated pulsed signals are attenuated. The Multiple Access feature of CDMA implies that each channel (as defined by frequency band, cell and PN code sequence) can be shared by Mobile Stations through assignment of a series of "time slots" by the Base Station. It is, accordingly, also a Time Division multiplexing scheme.
This invention makes use of the pulse and timing signals normally present in digital cellular Mobile Stations using Code Division Multiple Access (CDMA) signaling techniques. Once it has completed the Acquisition sequence, the Mobile station (MS) maintains its transmissions in accurate synchronism with the signals intended for it and the other co-channel MS's it receives steadily from the BS. Therefore, the BS can determine, from the relationship between its own timing and that of the received signals from a MS, the two-way transit time of the radio signals, and from it the "radio range". This range measurement may be greater than, but not less than the physical range, if for example the signal energy traveled by a non-straight path between the two stations. Since the Base Station emits signal bursts almost continuously, and an actively communicating Mobile Station transmits a signal burst every few milliseconds, many range measurements are possible each second. Because timing is tracked in the Base Station, range is available at any time. Because there is noise in the communications, there are corresponding variations in the measured range. These can be integrated (filtered) out by well known methods. At an effective signal to noise ratio of 10 db per Walsh Symbol (whose duration is 200 microseconds), integration time of 2 milliseconds will yield a calculated one-way range accuracy of approximately 50 feet, or at 200 millisecond integration time, range accuracy of about 5 feet.
Range measurement is enabled without modification of the sending or mobile station equipment because of the synchronization between received epochs and transmitted ones that are required for normal operation in digital telephony. The range measurement is made from a base station currently in contact with the mobile station by measuring the time interval from the start of its own transmitted pulse epoch to the start of a pulse epoch subsequently received from the mobile station.